AN-707 AN04653 - Datasheet Archive

 

APPLICATION NOTE One Technology Way · P.O. Box 9106 · Norwood, MA 02062-9106 · Tel: 781/329-4700 ·

 

AN-707 AN-707 APPLICATION NOTE One Technology Way · P.O. Box 9106 · Norwood, MA 02062-9106 · Tel: 781/329-4700 · Fax: 781/326-8703 · www.analog.com iCoupler® ESD and Latch-Up Considerations by Ronn Kliger Analog Devices' family of iCoupler digital isolators provides an alternative isolation solution to the traditional approach involving optocouplers. In general, iCoupler products offer performance, power consumption, size, reliability, and cost advantages relative to optocouplers. With their digital interfaces, iCoupler products are easier to use than optocouplers, negating the need to drive current into an LED, bias an output transistor, or deal with a varying or poorly constrained current transfer ratio. However, because iCoupler devices are fabricated using a high speed CMOS semiconductor process, care must be taken to avoid latch-up or ESD damage. This application note provides guidance and recommendations on these topics. INTRODUCTION Under typical conditions, iCoupler products are no more susceptible to ESD damage or latch-up than other CMOS components. However, since iCoupler products involve connections to circuitry with differing commonmode voltages, additional care must be taken to avoid ESD damage or latch-up. Table I shows a summary of the ESD testing that the ADuM130x/ADuM140x family was subjected to during product qualification. Table II contains a summary of the latch-up testing that the ADuM130x/ADuM140x family was subjected to during product qualification. Additional details can be found in the ADuM130x or ADuM140x Reliability Report. Table I. ADuM130x/ADuM140x ESD Qualification Results ESD Model Highest Pass Voltage (V) First Fail Voltage (V) Field-Induced Charged Device Model 1500 2000 Human Body Model 3500 4000 Machine Model 200 400 REV. 0 Table II. ADuM130x/ADuM140x Latch-up Qualification Results Condition Pulse Magnitude Current Pulses (50 s rise time, 5 ms duration) Voltage Pulses (50 s rise time, 5 ms duration) +102 mA, ­100 mA 9.75 V When an iCoupler is incorporated into a system design, it is possible for the above conditions to be exceeded inadvertently due to interactions between the system and the iCoupler device. Figure 1 shows a generalized diagram of an iCoupler device in a system along with the various stray capacitances C1, C2, C3, and C4 that are present. Also shown are bypass capacitors (C BP1 and CBP2) recommended for use with iCoupler products. If stray capacitances are too large or accounted for improperly, they can cause ESD damage or latch-up on an iCoupler device. This is discussed in detail in the ESD and Latch-Up sections that follow. Figure 1. An iCoupler Device in a System with Stray and Bypass Capacitances AN-707 AN-707 Using the terms of Figure 2, the total stray capacitance is given by: LATCH-UP Unlike most optocoupler devices, iCoupler products have CMOS interfaces. As a result, precautions should be taken (as with any CMOS component) to avoid a latch- up condition. The ADuM130x/ADuM140x Data Sheets cite a voltage of 7.0 V as an absolute maximum rating on the supply pins to avoid latch-up. Such a voltage could be generated either by noise on the supply lines or by excessive stray capacitances present during electrical testing. In both cases, good bypassing (CBP1 and CBP2) helps mitigate these effects. A value of between 0.01 F and 0.1 F is recommended, with these capacitors placed as close as possible to the iCoupler component. CSTRAY = C4 + CBP2 || C1 If CSTRAY is small compared to CBP1, the induced voltage is negligible. However, if CSTRAY is large (due to system shielding perhaps or a poor board layout) or CBP1 is small (due to inadequate bypassing), several volts can be induced at the VDD1 pin. This voltage is incremental to the dc supply voltage and can cause the ADuM140x to latch up. The precautions to take against this possibility are · · Figure 2. iCoupler Device with Stray Capacitances in the Presence of a Surge Test Voltage The stray capacitances shown in Figure 2 between the system ground and the ADuM140x's VDD1 pin provide a path for a fraction of the test voltage to get induced on the VDD1 pin. The simplified model is shown in Figure 3. BYPASS CAPACITORS The decoupling capacitors of Figure 1 (CBP1 and CBP2) have already been discussed in the context of avoiding latch-up. Their use is also strongly recommended to minimize supply voltage perturbations due to the iCoupler device's internal operation. An iCoupler device transmits short pulses internally to its transformers on every input logic transition. Each time one of these pulses is generated, the external supply experiences a sharp change in load and can experience a perturbation as a result. The use of adequate bypassing helps mitigate against this effect. Ensure supply voltages are as free as possible of noise and spikes ESD Although iCoupler devices contain ESD protection circuitry, appropriate ESD precautions should nonetheless be employed to avoid ESD -related damage. Furthermore, appropriate decoupling between iCoupler and system grounds should be employed to prevent systemlevel ESD transients from affecting the iCoupler device. Such decoupling typically involves adding inductance in the path between the system and iCoupler grounds. A more extreme solution involves the use of transient voltage suppressors known as TranZorbs. The key is to ensure the high, fast transients that the system chassis may experience are not imposed directly on the iCoupler device or across the isolation barrier. Provide adequate bypassing as close as possible to the iCoupler device's pins · Figure 2 shows the effect of stray capacitances when the system is subjected to electrical testing, such as the surge test of IEC 61000- 4-5. This test calls for an ac test voltage of up to 4 kV. The impact of excessive stray capacitance between the ADuM140x and the system ground can be to induce an incremental voltage on the ADuM140x's supply pin that could cause latch-up. Minimize the stray capacitances between an iCoupler device and the system ground as well as stray capacitance across the iCoupler device's isolation barrier Figure 3. Simplified Model Showing Effect of Stray Capacitances ­2­ REV. 0 AN-707 AN-707 Such bypassing should be placed as close as possible to the iCoupler component. A value of between 0.01 F and 0.1 F is recommended, as is the use of low ESL (equivalent series inductance) capacitors such as ceramic capacitors. It should be noted that many iCoupler products have two ground pins on one or both sides. When present, these are internally connected to each other via the lead frame. It is strongly recommended that both pins be externally connected to each other to minimize any impact of lead/bond wire inductances. The user should observe the following precautions when using iCoupler products: · · REV. 0 Ensure supply voltages are as free as possible of noise, spikes, and so on · ­3­ Provide adequate bypassing as close as possible to the iCoupler device's pins · CONCLUSIONS Analog Devices' iCoupler components have been qualified to ESD and latch-up criteria commonly found with CMOS devices. However, since iCoupler products are isolation devices bridging a galvanic barrier between two common-mode voltages, it is possible for systemlevel effects to cause ESD damage or latch-up. This could be caused by excessive stray capacitances across the isolation barrier, inadequate decoupling between iCoupler and system grounds, inadequate bypassing, or a combination of these factors. Minimize the stray capacitances between an iCoupler device and the system ground as well as stray capacitance across the iCoupler device's isolation barrier Provide adequate decoupling between iCoupler device and system grounds AN04653 AN04653­0­2/04(0) © 2004 Analog Devices, Inc. All rights reserved. Trademarks and registered trademarks are the property of their respective owners. ­4­